1. Field of the Invention
The present invention relates to piezoelectric resonators used as various kinds of resonators and oscillators, and piezoelectric resonance components incorporating such resonators. More specifically, the present invention relates to thickness-extensional piezoelectric resonators utilizing harmonics of a thickness-extensional vibration mode and piezoelectric resonance components incorporating such resonators.
2. Description of the Related Art
Piezoelectric resonators have been used in piezoelectric resonance components such as a piezoelectric oscillator and a piezoelectric filter. It is known that such conventional piezoelectric resonators use different piezoelectric resonance modes according to frequencies used.
In Japanese Unexamined Patent Publication No. 1-117409, an energy-trap type piezoelectric resonator utilizing the second-order harmonic of the thickness-extensional vibration mode is disclosed. This piezoelectric resonator will be described below referring to FIGS. 9 and 10.
FIG. 9 is an exploded perspective view of the piezoelectric resonator. In FIG. 9, the piezoelectric resonator is produced by integrally firing two laminated piezoelectric ceramic green sheets 51 and 52. A round excitation electrode 53 is provided at the center of the ceramic green sheet 51 and is led out to an edge of the ceramic green sheet 51 via a leading electrode 54. In addition, a round excitation electrode 55 is provided at the center of the upper surface of the ceramic green sheet 52 and is led out to an edge of the ceramic green sheet 52 via a leading electrode 56. An excitation electrode 57 is provided on the lower surface of the ceramic green sheet 52 and is led out to an edge of the ceramic green sheet 52 via a leading electrode 58. This is indicated by a reflection of the configuration below the ceramic green sheet 52.
Pressure is applied to the laminated ceramic green sheets 51 and 52 in the thickness direction of the laminated structure so as to obtain a fired body. Subsequently, polarization processing is performed on the fired body so as to produce a piezoelectric resonator 60 shown in FIG. 10.
In the piezoelectric resonator 60, polarization processing is performed on piezoelectric layers 61 and 62 in a direction indicated by arrows shown in FIG. 10. That is, polarization processing is performed on the fired body uniformly in the thickness direction of the structure.
In the case of driving, the excitation electrodes 53 and 57 are commonly connected and an AC voltage is applied between the excitation electrodes 53, 57, and 55 so as to vibrate the piezoelectric resonator 60. Excitation energy is trapped in a region where the excitation electrodes 53, 55, and 57 overlap, that is, in a resonating portion A.
As described above, the piezoelectric resonator 60 using a harmonic of the thickness-extensional vibration mode defines an energy-trapping-type piezoelectric resonator. Thus, it is necessary to dispose a vibration-attenuating portion for attenuating vibration at an area surrounding the resonating portion A. In other words, a vibration-attenuating portion that is larger than the area of the resonating portion needs to be provided. As a result, it is very difficult to reduce the size of the conventional piezoelectric resonator 60.
Japanese Unexamined Patent Publication No. 2-235422 discloses another energy-trap type piezoelectric resonator using a strip-type piezoelectric ceramic body. In the piezoelectric resonator, it is not essential to provide an extra piezoelectric-substrate portion around the resonating portion. As shown in FIG. 11, an excitation electrode 72a is provided on the upper surface of a narrow piezoelectric substrate 71, and an excitation electrode 72b is provided on the lower surface thereof. The excitation electrodes 72a and 72b are formed such that their widths are equal to the width of the piezoelectric substrate. The excitation electrodes 72a and 72b oppose each other at the center in the length direction of the piezoelectric substrate 71 to define a resonating portion. In addition, the excitation electrodes 72a and 72b are arranged to extend to opposite edges 71a and 71b in the length direction of the piezoelectric substrate 71.
In a piezoelectric resonator 70 shown in FIG. 11, in the case of excitation of the thickness-extensional vibration mode, unnecessary vibrations occur due to the dimensional relationship between the width W and the thickness T of the piezoelectric substrate 71. Regarding this problem, in Japanese Unexamined Patent Publication No. 2-235422, it is shown that, in order to reduce unnecessary spurious vibrations between a resonant frequency and an anti-resonant frequency while a fundamental wave is used, the value of W/T must be substantially equal to 5.33 in the 16 MHz resonant frequency. In the event the third-order harmonic is used, the value of W/T must be substantially equal to 2.87 in the 16 MHz resonant frequency.
In the energy-trap type piezoelectric resonator disclosed in Japanese Unexamined Patent Publication No. 2-235422, since it is not necessary to provide a vibration-attenuating portion around the resonating portion, the size of the piezoelectric resonator can be reduced. However, when the harmonic of the thickness-extensional vibration mode is actually used, in addition to spurious vibrations generated between a resonant frequency band and an anti-resonant frequency band, various undesirable spurious vibrations occur, with the result that effective resonance characteristics cannot be obtained. Furthermore, the electric capacity of this piezoelectric resonator is relatively small, the resonator is thereby susceptible to influence from the stray capacitance of a circuit board.